1. Field of the Invention
The present invention relates generally to fiber optics. In particular, the present invention relates to polarization beam splitters and combiners.
2. The Prior Art
In the field of fiber optics, one of the most valuable properties of light is the phenomenon of polarization. Light is described as a transverse wave when travelling through a medium such as glass, air or vacuum, whereby by the electric and magnetic fields which comprise the light oscillate in a plane perpendicular to the direction in which the light is travelling. Many factors may influence the polarization of light, including reflections from surfaces, external magnetic fields, and in particular, stresses in the transmitting media.
One application that results from the polarization of light or laser energy is the ability to combine multiple energy sources for a higher total power in an application known as polarization multiplexing. Polarization multiplexing results in higher total output power as well as lower polarization-dependent gain from an amplifier such as a Raman or EDFA type amplifier. When performing polarization multiplexing, typically a polarization beam splitter/combiner (PBS) is used.
A PBS is a device which either combines or splits multiple light sources according to the polarization direction of the light. A typical PBS comprises a common port and two splitting ports or combining ports. The common port is usually comprises a SM fiber and the two splitting or combining ports usually comprise PM fibers. In some special cases, the common port may be a PM fiber and the splitting or combining ports may be SM fibers. When a polarization beam splitting function is performed, light comes from the common port will be split into two orthogonal polarization components, which are subsequently received by the two combining or splitting ports.
As is known by those skilled in the art, it is usually preferable to have the slow axis or fast axis of receiving PM fibers match the splitted light polarization directions to minimize insertion losses. When a polarization beam combining function is performed, light coming from the two combining ports is linearly polarized orthogonal to each other, combined by a polarization beam splitter/combiner prism, and received by the common port. When the polarization direction of light coming from the combining port is not aligned with its respective direction from the polarization splitting operation, only the component that along the splitting polarization direction will be directed to the common port, an the other orthogonal component will be directed to a different undesired optical path resulting a high combining loss. For example, a typical pump laser for EDFA or Raman amplifier usually has a PM fiber output and the laser light coming out of the PM fiber is usually linearly polarized with its polarization direction being along the slow axis of the PM fiber. When the light coming out of the PM fiber is incident upon the polarization beam splitter/combiner, it is essential to have its polarization direction match the principal polarization axis of the polarization beam splitter/combiner to avoid any beam splitting which may result in substantial insertion loss.
Thus it is a challenge to manufacture a polarization beam splitter/combiner with its principal polarization axis matching a preferred orientation of PM fiber, such as the slow or fast axis.
FIG. 1 shows a functional block diagram of a prior art PBS. The following disclosure will discuss PBS 100 in the splitter mode of operation, but the process may be reversed for a combiner operation.
PBS 100 includes a fiber 102 configured to receive incident light launched into PBS 100. Fiber 102 is optically coupled to a first lens 104 configured to pass incident light to a prism 108. Prism 108 may comprise a polarizing beamsplitter cube, a Glan-Foucault prism, or other suitable prism known in the art.
Prism 108 is configured to transmit light of a first polarization direction, generally known as the P direction, through a second lens 112 on to a fiber 114. Fiber 114 may comprise a polarization maintaining fiber (PM) fiber or SM fiber standard in the art. Prism 108 is also configured to reflect light of a second polarization known as S polarization through a third lens 118 to a fiber 120. Fiber 120 may be composed of either a PM fiber or a SM fiber.
In operation, incident light having an arbitrary polarization emitting from fiber 102 is coupled through a first lens 104 onto the prism 108. The arbitrary polarization direction is shown in FIG. 1 as light of a first polarization being represented by an arrow, and light of a second polarization being represented by an X. As is known by those skilled in the art, light with an arbitrary polarization direction usually has polarization components in both S and P directions. When the incident light 122 passes through the prism 108, light of a first polarization (P polarization) is transmitted through a second lens 112 to a fiber 114. Light of a second polarization (S polarization) is reflected in prism 108 through a third lens 118 on a fiber 120.
In the case where fibers 114 or 120 are PM fibers, fiber 114, or fiber 114 and lens 112 together, may be rotated with respect to the axis 110 to align the slow axis or fast axis of PM fiber 114 or a preferred orientation to the polarization direction P. Similarly, fiber 120, or fiber 120 and third lens 118 together, may be rotated with respect to the axis 116 to align the slow axis or fast axis of PM fiber 120 or a preferred orientation to the polarization direction S.
While the PBS of FIG. 1 works for its intended purpose, certain disadvantages have been noted. For example, the configuration of FIG. 1 results in a three-port apparatus utilizing three lenses. This three-port, three lens configuration results in a relatively large size. In addition, it increases manufacturing and packaging complexity since three independent ports have to be aligned and packaged.
Therefore, there is a need for a smaller sized polarization beam splitter and combiner with a reduced number of components.
Furthermore, there is a need for a polarization beam splitter/combiner that is smaller than those of the prior art.
Additionally, prior art devices typically place two PM fiber adjacent to each other and a SM fiber on the opposite end to achieve a compact package. However, this design requires precise alignment of each PM slow axis with respect to each other, polarization direction matching of fiber to each light component can not be independently adjusted. Thus performance is poor and yield is low.
Hence there is a need to achieve a compact package while providing independent adjustment of polarization direction matching.
The invention satisfies the above needs. The present invention relates generally to fiber optics. In particular, the present invention relates to polarization beam combiners and splitters.
A method for producing an optical device according to the present invention is disclosed which comprises providing a first fiber; optically coupling the first fiber to a common lens, and optically coupling the common lens to a polarization dependent reflector; optically coupling a second fiber to the polarization dependent reflector through the common lens, the first and second fibers being disposed adjacent to one another; optically coupling a third fiber to the polarization dependent reflector through a second lens; and aligning the device such that transmitted light is provided to the first and second optical fibers through the common lens, and the reflected light is provided to the third optical fiber through the second lens.
An optical device according to the present invention is disclosed which comprises a first optical fiber and a second optical fiber disposed adjacent to each other, the first fiber being optically configured to receive incident light; a polarization dependent reflector having a preferred axis; a common lens configured to optically couple the first and second fibers to the polarization dependent reflector; and a third optical fiber coupled to the polarization dependent reflector through a second lens. The device may be characterized in that the optical device is aligned such that incident light launched into the polarization dependent reflector is provided to the second fiber at both a predetermined angle and a predetermined polarization direction; and such that incident light launched into the polarization dependent reflector is launched into the third fiber at a predetermined polarization direction.
Various aspects of the present invention are disclosed, including where the first and third optical fibers are disposed on opposite sides of the polarization dependent reflector along the preferred axis of the polarization dependent reflector. A further aspect is disclosed wherein the first or said second optical fibers comprise polarization maintaining fibers. Yet another aspect is disclosed wherein the predetermined angle comprises an angle between approximately 2xc2x0 and approximately 5xc2x0.
Further aspects of the present invention are disclosed wherein the predetermined polarization of the reflected light or transmitted light substantially overlaps the slow or fast axis of the second or third optical fiber respectively.